This invention relates to the field of nondestructive testing of objects by interferometry. The invention provides a method and apparatus for perturbing the object being tested, without mechanical coupling between the source of excitation and the object. It is especially useful for testing large objects, and also for testing manufactured articles which may be subject to delamination.
The invention is especially useful in testing so-called "honeycomb" structures used in aircraft. Such structures include a core having cells of hexagonal cross-section (hence the name "honeycomb"), and can be made of metal or ceramic materials. The honeycomb core is typically covered by a "skin" made of aluminum, Fiberglas, paper, graphite, or other material. Also, such honeycombs can include layers of honeycomb cores separated by layers of such skins. The present invention can be used to test such objects for delamination of the skin, and/or disassembly of the honeycomb structure itself.
Nondestructive testing is the detection and analysis of defects in an object, wherein the object is not damaged by the test. Conventional methods of nondestructive testing have included bombarding an object with ultrasonic waves, and detecting the changes in amplitude of the waves as they travel through the object or as they are reflected by a defect within the object. The latter method has the disadvantage that a water medium is generally required to conduct the ultrasonic waves to the object. Therefore, the method requires wetting the object.
Another method of nondestructive testing is radiography, which includes passing x-rays through the object. Radiography is useful in determining whether there is a cavity within the object, but it is not generally useful for detecting delaminations in composite structures or in the honeycomb panels mentioned above. Radiography cannot detect "unbonds" in composite structures unless the x-rays happen to be directed in exactly the correct plane.
Another method of nondestructive testing is shearing interferometry, or "shearography". In shearography, interferograms are formed by superimposing two laterally displaced ("sheared") images of the same object. In shearography, one compares an interferogram, or "shearogram", taken while the object is not stressed with similar interferograms taken while the object is stressed. Examples of methods of shearography are given in U.S. Pat. Nos. 4,139,302 and 4,887,899, the disclosures of which are incorporated herein.
In the prior art, a vacuum is commonly used as the means of stressing the object being tested. A relatively small test object such as a honeycomb panel can be rapidly and reliably deformed with a vacuum, for purposes of nondestructive testing. But if that object is installed on an aircraft, it may be very difficult to remove for testing. In such cases, vacuum stressing is impractical and unsatisfactory.
Another example of the shortcomings of vacuum stressing is in the analysis of the foam insulation on a cryogenic fuel tank of a space vehicle. The size of such an object precludes the use of a vacuum test chamber. Moreover, for soft materials such as foam, vacuum stressing can damage the object being tested. Vacuum stressing is often performed with a vacuum window, which can be a flat piece of clear plastic having a seal around its perimeter, and which, when pressed against the object, forms a vacuum chamber having one side defined by the object. The vacuum window may cause local damage at the point of contact. Also, many large structures used in the aerospace industry are not easily accessible, even when a vacuum window is used. In many cases, an inordinate amount of labor is required to position the nondestructive testing equipment.
It has also been known to perturb an object by vibration. U.S. Pat. No. 4,408,881 describes the vibrational excitation of a test object using a random frequency ("white noise") generator, and using mechanical coupling between the source of excitation and the test object. While the latter technique is useful when it is necessary to detect small delaminations or "unbonds", having a principal dimension of about 1/8 inch or smaller, the technique has the disadvantage that it requires excitation of the entire object. Thus, the greater the weight of the test object, the greater the energy required to vibrate the object.
Another problem with the above-described method is that it is very difficult to design a mechanical coupling device which will uniformly excite a large test object. Test objects having a principal dimension of 10 inches or larger are generally excited more at the point of attachment of the mechanical coupling than at other points. This nonuniformity makes it difficult to interpret the resulting interferogram. Thus, while vibrational excitation by mechanical coupling is useful with small test objects, it cannot be used effectively for determining the mechanical integrity of aircraft or other large structures.
The present invention solves the problem of nondestructively testing a large and heavy object, by providing a method and apparatus for air-coupled acoustic excitation of the object. The invention is especially suitable for detecting delaminations near the surfaces of composite structures. It is also an effective means of testing a large object which is difficult to vibrate, and which cannot be easily mechanically coupled to a testing apparatus.